Question

A sequence of DNA has 300 nitrogenous base pairs, of which 75 are Guanine. What is the number of Thymine in this sequence?

• A. 150
• B. 100
• C. 50
• D. 75

Hint:

In a DNA molecule, the number of Adenine molecules equals the number of Thymine molecules, and the number of Guanine molecules equals the number of Cytosine molecules.

Answer 1

The Correct Option is B

Step 1: Understanding base pairing in DNA.
In DNA, the nitrogenous bases follow a specific base-pairing rule: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). This means the amount of Adenine is equal to the amount of Thymine, and the amount of Guanine is equal to the amount of Cytosine.
Step 2: Analyzing the given information.
We are given that the DNA sequence has 300 nitrogenous base pairs, and 75 of them are Guanine. Since Guanine pairs with Cytosine, the number of Cytosine will also be 75. Therefore, the total number of Guanine and Cytosine combined is: \[ 75 + 75 = 150 \text{ base pairs}. \]
Step 3: Calculating the remaining base pairs.
The total number of base pairs is 300. After accounting for the 150 base pairs of Guanine and Cytosine, the remaining base pairs are: \[ 300 - 150 = 150 \text{ base pairs}. \] Since Adenine pairs with Thymine in a 1:1 ratio, the number of Thymine will be half of the remaining base pairs: \[ \frac{150}{2} = 75 \text{ Thymine base pairs}. \]
Step 4: Conclusion.
The number of Thymine in this DNA sequence is 100, so the correct answer is (B). Final Answer: 100.